Adaptive control for zero-voltage switching in a multi-switch switching power converter

ABSTRACT

A switching power converter is provided that adaptively changes the on-time period for an auxiliary switch transistor to locate a boundary between sufficient and insufficient energy.

TECHNICAL FIELD

This application relates to switching power converters, and moreparticularly to an adaptive control for zero-voltage switching in amulti-switch switching power converter.

BACKGROUND

The high efficiency of switching power converters such as flybackconverters has led to their virtual universal adaption as the batterycharger for mobile devices. In a flyback converter, a primary-sidecontroller controls the cycling of a power switch transistor thatconnects between the transformer's primary winding and ground. Arectified AC mains voltage drives the primary winding current when thepower switch is cycled on. The rectified AC mains voltage can be severalhundred volts such that it can stress the power switch transistor. Tominimize the switching stress for the power switch transistor, bothquasi-resonant (valley-mode switching) and zero-voltage switchingtechniques are known. For example, it is known to employ valleyswitching techniques with regard to the resonant oscillation of thedrain voltage for the power switch transistor when it is cycled off. Thepeak voltages for the resonant oscillation can be relatively robust (asmuch as 200 V or higher) whereas the minimum voltages (the valleys inthe resonant oscillations) are much lower. Valley-mode switching thusinvolves the detection or prediction of a particular valley in theresonant oscillations so that the power switch transistor may beswitched on at the time of the particular valley.

Although valley-mode switching thus lowers the voltage stress on thepower switch transistor, note that the valley voltages are not zero butmay range from 20 V or even higher such as 60 V. This relatively highdrain voltage is then discharged to ground when the power switchtransistor is cycled on, which lowers efficiency. A more power-efficientalternative to valley-mode switching is zero-voltage-switching (ZVS). InZVS operation, the leakage energy in the transformer is stored andreclaimed in a capacitor that is coupled to the drain voltage of thepower switch transistor through an active clamp switch. The active clampswitch is cycled on at the peak of the resonant oscillations, whereuponthe drain voltage is discharged below ground as the leakage energy isreclaimed. An ZVS architecture thus has no stressing switches at theon-time of the power switch transistor.

However, the detection of the zero-voltage switching point has so farproven to be problematic. In particular, it is conventional to calculatethe circuit energy so as to estimate the needed energy to complete ahalf-cycle of resonant oscillation. But such an estimation reliesheavily on the accuracy of the circuit parameters and is thus subject toconsiderable process variation. Moreover, the half-cycle estimation islengthy and consumes substantial calculation power. The resultinginaccuracies result in either a hard turn of the power switch or wasteof resonant energy and large voltage stress.

Accordingly, there is a need in the art for improved control ofzero-voltage switching for switching power converters.

SUMMARY

A technique to achieve optimal zero-voltage switching in provided for aswitching power converter and/or a boost converter that includes a powerswitch transistor connected to an inductive storage element. In anon-isolated switching power converter such as a buck converter, theinductive storage element is an inductor whereas it is a primary windingof a transformer in an isolated switching power converter such as aflyback converter. Regardless of whether the switching power converterdrives an isolated load or not, the power switch transistor allows apositive current to flow through the inductive storage element when thepower switch transistor is turned on in a switching cycle. The powerswitch transistor then cycles off so that the stored energy in theinductive storage element may be delivered to the load. To achieveoptimal zero-voltage switching, the techniques and systems disclosedherein adapt the on-time of an auxiliary switch transistor. Theauxiliary switch transistor is cycled on while for an adaptive on-timeperiod after the power switch transistor is cycled off. When theauxiliary switch transistor is switched off following its adaptiveon-time, a negative current is induced in the inductive storage element.

In a flyback converter, the negative current discharges the drainterminal for the power switch transistor. If the adaptive on-time periodis relatively short, the drain terminal does not discharge to ground butinstead discharges to a positive local minimum and begins resonantlyoscillating. If the adaptive on-time period is relatively long, thedrain terminal discharges to ground. Since the source terminal isgrounded for a flyback converter's power switch transistor, thedrain-to-source voltage for the power switch transistor is zero voltswhen the drain terminal is grounded so that it may be switched on usingzero-voltage switching. Conversely, the negative current charges thesource terminal in a buck converter. The drain terminal is charged tothe input voltage. Thus, if the adaptive on-time period is relativelylong, the source terminal may be charged to the input voltage, whichmakes the drain-to-source voltage for the buck converter's power switchtransistor to be zero volts so that it may be switched on usingzero-voltage switching. If the adaptive on-time period is relativelyshort, the drain-to-source voltage for the buck converter's power switchtransistor would instead resonantly oscillate through a series ofpositive voltage valleys (local minima).

Relatively short adaptive on-time periods that result in valley-modeoscillations of the drain terminal voltage that do not have a magnitudesufficient to make the drain-to-source voltage zero volts produce acondition denoted herein as insufficient energy. In contrast, relativelylong adaptive on-time periods that result in a zero drain-to-sourcevoltage for the power switch transistor produce a condition denotedherein as sufficient energy. But note that as the adaptive on-timeperiod is extended, the energy becomes “too sufficient” such that thedrain-to-source voltage is pulled below zero rather than just to zero.Switching on the power switch transistor with such a negativedrain-to-source stresses the transistor and lowers efficiency. But priorart zero-voltage switching techniques never recognized that there is anoptimal boundary between the inefficient and efficient energy conditionsthat produce the optimal zero-voltage switching conditions. Thezero-voltage switching technique disclosed herein adapts the adaptiveon-time period so that the switching power converter operates at theboundary between the sufficient and insufficient energy conditions. Theresulting control is quite advantageous with regard to minimizingswitching stress and increasing efficiency.

These advantageous features may be better appreciated through aconsideration of the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a circuit diagram of an example active clamp flybackconverter configured for optimal zero-voltage switching in accordancewith an aspect of the disclosure.

FIG. 1B is a circuit diagram of a flyback converter with synchronousrectification configured for optimal zero-voltage switching inaccordance with an aspect of the disclosure.

FIG. 1C is a circuit diagram of a buck converter configured for optimalzero-voltage switching in accordance with an aspect of the disclosure.

FIG. 2 illustrates the effect of auxiliary on-time period variation onthe drain-to-source voltage and also on the detections of zero-crossingsand valleys for the drain-to-source voltage in accordance with an aspectof the disclosure.

FIG. 3 is a diagram of an example controller for implementing optimalzero-voltage switching in accordance with an aspect of the disclosure.

FIG. 4 is a circuit diagram of a type II valley-mode detector inaccordance with an aspect of the disclosure.

Embodiments of the present disclosure and their advantages are bestunderstood by referring to the detailed description that follows. Itshould be appreciated that like reference numerals are used to identifylike elements illustrated in one or more of the figures.

DETAILED DESCRIPTION

All isolated switching power converters and/or boost converters includea power switch transistor that, when switched on, allows a positivemagnetizing current to develop in a primary winding of a transformer.While the primary winding conducts, a secondary winding for thetransformer is prevented from conducting. The control of the secondarywinding current may be performed by an output diode but more efficientdesigns use a synchronous rectifier (SR) switch transistor forcontrolling when the secondary winding conducts. The SR switchtransistor is switched on while the secondary winding current flows andis then switched off. In a discontinuous conduction mode of operation,the switching off of the SR switch transistor induces a negativemagnetizing current in the primary winding that lowers the drain voltagefor the power switch transistor. The amount of discharge of the drainvoltage for the power switch transistor depends upon the synchronousrectifier switch on time. Should the on time be relatively short, thepower switch transistor's drain voltage does not discharge all the wayto ground but instead resonantly oscillates through a series of localminima denoted as valleys. These valleys are exploited in valley-modeswitching modes of operation as the appropriate switch on times for thepower switch transistor to minimize the switching stresses and loss. Butif the synchronous rectifier switch is maintained for a greaterduration, the drain voltage for the power switch transistor isdischarged to ground. Since the source is grounded, the drain-to-sourcevoltage for the power switch transistor is zero voltage if the drainvoltage is discharged to ground. A zero-voltage switching mode ofoperation may then be implemented so that the power switch transistor iscycled on when its drain-to-source voltage is zero.

The synchronous rectifier switch on time thus controls whether the powerswitch transistor may be cycled using valley-mode switching orzero-voltage switching operation. But such control of the switching modefor the power switch through the on time duration of an auxiliary switchtransistor is not limited to flyback converters with synchronousrectification. As used herein, the term “auxiliary switch transistor” isdefined in conjunction with a power switch transistor that allows apositive current to flow through an inductive storage element when thepower switch transistor is cycled on. The auxiliary switch transistor isconfigured so that it induces a negative current in the inductivestorage element after the auxiliary switch transistor is cycled on for asufficiently long duration and then cycled off. With regard to a flybackconverter with synchronous rectification, the inductive storage elementis the primary winding and the positive current is the magnetizingcurrent. But there are numerous other types of switching powerconverters that include such an auxiliary switch transistor. Forexample, turning now to the drawings, FIG. 1A illustrates an activeclamp flyback converter 100 that includes a power switch transistor M1and also an active clamp switch transistor M2 that functions as anauxiliary switch transistor. During a power switch cycle, power switchtransistor M1 is cycled so that a magnetizing current builds up in aprimary winding T1 as powered by an input voltage source Vin. Thismagnetizing current flows from an input power rail for the primarywinding T1 into the drain node LX of power switch transistor M1. Whilepower switch transistor M1 is on, an output diode D is reversed biased.Output diode D is connected to a secondary winding T2 for the flybackconverter's transformer so that no secondary winding current flows whilethe power switch transistor M1 is conducting. When power switchtransistor M1 is switched off, output diode D becomes forward biased sothat the secondary current flows to charge an output voltage Vo acrossan output capacitor Co. A load (not illustrated) is powered by theoutput voltage (or the output current from secondary winding T2).

But the transformer is not perfect such that some of the magnetic energyin the primary winding T1 does not couple with second winding T2 butinstead charges a leakage inductance for primary winding T1 while powerswitch transistor M1 is on. This leakage energy is captured by an activeclamp capacitor Ca in series between active clamp switch transistor M2and the input voltage rail for the primary winding T1. The stored energyis then returned to the transformer by cycling active clamp switch M2 onwhile power switch M1 is off. Depending upon the on-time duration forthe active clamp switch transistor M2, the drain voltage on node LX maybe discharged to ground due to the negative current flowing through theprimary winding in response to the cycling off of the active clampswitch transistor M2. It may thus be appreciated that active clampswitch transistor M2 satisfies the definition provided earlier for anauxiliary switch transistor in that when active clamp switch transistorM2 is cycled on and then off, a negative current flows through primarywinding T1.

A synchronous rectifier (SR) flyback converter 105 as shown in FIG. 1B.Power switch transistor M1 is in series with primary winding T1 asdiscussed with regard to active clamp flyback converter 100. But theoutput diode D on the secondary side of the transformer is replaced byan SR switch transistor M3. While the primary switch transistor M1 ison, a secondary controller (not illustrated) would maintain SR switchtransistor M3 off and then cycle SR switch transistor M3 for an SR ontime on in response to the cycling off of the power switch transistorM1. The secondary current then pulses high and ramps down to zero at thecompletion of the SR on time (in a discontinuous conduction mode ofoperation), Should the SR on time be sufficiently long so that thesecondary current ramps down to zero, the output voltage Vo forces anegative secondary current to flow through secondary winding T1. Theresulting stored magnetic energy in the transformer then pulls the LXnode voltage low and forces a negative magnetizing current to flow fromthe LX node to the input power rail following the completion of theauxiliary switch on time. SR switch transistor M3 thus satisfies thedefinition herein of an auxiliary switch transistor.

Another example of an auxiliary switch transistor is given by a low-sidetransistor M5 in a buck converter 110 shown in FIG. 1C. Buck converter110 includes a high-side switch transistor M4 (Main Switch) that couplesbetween an input voltage node and an inductor L. During a power switchcycle, high-side switch transistor M4 is switched on to allow a positivecurrent to begin increasing through inductor L. After the desired peakcurrent is reached, high-side switch transistor M4 is switched off. Atthat point, low-side switch transistor M5 is switched on so that theinductor L may freewheel and charge an output capacitor Co with theoutput voltage. The positive current through inductor L will dischargeto zero and become negative if the on-time duration for low-side switchtransistor M5 is sufficiently long. When low-side switch transistor M5cycles off, the resulting negative current through inductor L may chargethe drain terminal for high-side switch transistor M4 to the inputvoltage Vin such that its drain-to-source voltage is zero. Low-sideswitch transistor M5 thus satisfies the definition of an auxiliaryswitch transistor as defined herein. Other examples of auxiliary switchtransistors are included in active clamp forward converters, buck/boostconverters with synchronized rectifiers, totem-pole bridgeless PFCconverters, half-bridge converters, and full bridge converters.

An adaptive control technique is provided herein that controls the ontime for the auxiliary switch transistor so as to develop an appropriateamount of negative current in the inductive storage element after theauxiliary switch transistor is cycled off. Depending upon the durationof the on time for the auxiliary switch transistor, the development ofthis negative magnetizing current forces the drain-to-source voltage forthe power switch transistor to fall to zero in a resonant oscillation.The resonant oscillation for the power switch transistor'sdrain-to-source voltage that does not cross zero volts (or a voltagesufficiently close to zero) is denoted herein as an “insufficientenergy” condition. An insufficient energy condition results from atoo-short of an on time for the auxiliary switch transistor. Incontrast, on times for the auxiliary switch transistor that result inthe drain-to-source voltage for the power switch transistor dropping toor below zero volts (or some threshold voltage sufficiently close tozero) result in what is denoted herein as a “sufficient energy”condition. Since zero voltage switching may be deemed to exist if thepower switch transistor is switched on at a sufficiently low voltage(close to zero volts), the threshold drain-to-source voltage for thepower switch transistor that satisfies zero voltage switching is denotedherein as the “zero voltage switching threshold voltage.” In general,the zero voltage switching threshold voltage is zero volts but it may beslightly positive (or negative) for alternative embodiments.

Regardless of whether the zero voltage switching threshold voltage iszero or non-zero, zero-voltage switching may thus be achieved for thepower switch transistor by ensuring a sufficiently long on time for theauxiliary switch transistor so that a sufficient energy condition isachieved. But a too-long of an on time for the auxiliary switchtransistor is also sub-optimal as the drain-to-source voltage for thepower switch transistor may then be pulled below zero volts before thepower switch transistor is cycled on. The resulting negativedrain-to-source voltage for the power switch transistor when cycled oncauses switching stress and wastes energy analogous to the sameundesirable effects that occur with valley switching of the power switchtransistor. Advantageously, the zero voltage switching techniquedisclosed herein operates at the boundary between insufficient andsufficient energy such that optimal zero voltage switching is achieved.In this fashion, power efficiency is maximized and power switchingstresses are minimized.

The technique for locating the boundary between sufficient andinsufficient energy for zero voltage switching of the power switchtransistor depends upon whether a switching power converter includes avalley-mode detector and/or a zero-crossing detector. As known in thevalley-mode switching arts, a valley-mode detector detects the valleysor local minima in the resonant oscillation of the power switchtransistor's drain voltage following the cycling off of the power switchtransistor. A conventional valley-mode detector can only detect valleysand does not detect minima that equal zero volts or less. Such avalley-mode detector is designated herein as a type I valley-modedetector. A more sophisticated valley-mode detector will be discussedfurther herein that can detect all minima such that it detects minimathat equal zero volts or less. Such a valley-mode detector is designatedherein as a type II valley-mode detector.

The effects on the drain-to-source voltage (Vds) for a power switchtransistor from varying the adaptive on-time period for the auxiliaryswitch transistor may be better appreciated with reference to FIG. 2. Afirst adaptive on-time period 200 is relatively long and results in Vdsdropping to zero at time t0. A second adaptive on-time period 205 isshorter than the period 200 and results in Vds dropping to zero at atime t1. A third adaptive on-time period 210 is shorter than period 205and results in Vds dropping to zero at time t2. But note that a furtherreduction in the on-time period as shown for adaptive on-time period 215results in a positive valley 220 for Vds at time t3 rather than a zerocrossing. Adaptive on-time period 215 is thus resulting in an optimalzero-voltage switching at the boundary between sufficient energy zone225 and insufficient energy zone 230.

With regard to these adaptive on-time periods, note that a zero-crossingdetector (ZCD) 235 would detect the zero crossings at times t0, t1, andt2. But there is no response for ZCD 235 to valley 220. In contrast, atype II valley detector 240 not only detects the zero crossings at timest0, t1, and t2 but also detects valley 220 at time t2. Finally, a type Ivalley detector 245 only detects valley 220. Some example controllersthat respond to these valley and zero-crossing detections by varying theadaptive on-time period to achieve optimal zero-voltage switching at theboundary between sufficient and insufficient energy will now bediscussed. An example controller 300 is shown in FIG. 3. A valleydetector 305 detects valleys in an input voltage Vin and transmits thevalley identifications to a adaptive on-time control circuit 315.Similarly, a ZCD 310 detects zero crossing for the input voltage Vin andtransmits the zero crossing identifications to adaptive on-time controlcircuit 315.

Adaptive on-time control circuit 315 varies the adaptive on-time periodfor an auxiliary switch transistor to enable optimal zero-crossingswitching of a power switch transistor (not illustrated). Operation ofadaptive on-time control circuit 315 for an implementation in whichvalley detector 305 is a type I valley detector will be discussed first.Should the adaptive on-time period be such that only valleys aredetected, adaptive on-time control circuit 315 gradually increases theadaptive on-time until valleys are no longer detected but instead onlyzero-crossing detections are made. Adaptive on-time control circuit 315thus detects the boundary between sufficient and insufficient energy sothat optimal zero-voltage switching may ensue. Should the adaptiveon-time period be such that only zero crossings are detected, adaptiveon-time control circuit 315 may shorten the adaptive on-time perioduntil valleys are detected. By again increasing the adaptive on-timeperiod just until valley cease and zero crossings are detected, theboundary between sufficient and insufficient energy is detected. Notethat adaptive on-time control circuit 315 may operate without ZCD 310yet still enable zero-voltage switching through the use of a time 320.In particular, adaptive on-time control circuit 315 may respond to thedetection of a valley by extending the adaptive on-time period by anincrement of time and again observing whether valleys are detected. Theadaptive on-time period may thus be extended until no valleys aredetected. A timer 320 may thus trigger adaptive on-time control circuit315 to switch on of the power switch transistor (not illustrated) inresponse to the expiration of a time-out period. The time-out period maybe adaptively adjusted in alternative embodiments.

Operation of adaptive on-time control circuit 315 with a type II valleydetector 305 will now be discussed for an embodiment that does notinclude ZCD 310. Note that the time gap between the zero-crossing timesfor sufficient energy cases as a function of a increment of time for theadaptive on-time period may be observed by adaptive on-time controlcircuit 315. In the insufficient energy regime, this time gap is quitesmall since the resonant oscillation frequency is fairly stable. But atthe boundary with sufficient energy operation, the time gap will becomerelatively large. Adaptive on-time controller 300 may thus observe thetime gap and detect the optimal zero-crossing condition without the useof ZCD 310. Alternatively, ZCD 310 may be used in conjunction with atype II valley detector 305.

In addition, note that adaptive on-time control circuit 315 may operatewith only ZCD 310. Beginning with sufficient energy operation, adaptiveon-time control circuit 315 may progressively reduce the adaptiveon-time period and observe the gap increase between the correspondingzero crossings. The gap will increase until insufficient energy isreached, whereupon there are no more zero crossings. In such a case,timer 320 may time out and trigger a power switch cycle. Adaptiveon-time control circuit 315 may thus determine the maximum gap prior toinsufficient energy operation and set the adaptive on-time period foroptimal zero-crossing accordingly. Referring again to FIGS. 1A, 1B, and1C, controllers 101, 106, and 115 may operate as discussed with regardto controller 300.

A type II valley mode detector 400 shown in FIG. 4 will now bediscussed. In this embodiment, a node lx is the drain voltage for powerswitch transistor M1 of FIG. 1A. This drain voltage may be relativelyhigh so it is divided through a capacitive voltage divider formed by aclamp transistor M6 and a capacitor C1 to form a divided drain voltagelxc. To filter low-frequency noise on the divided drain voltage lxc,voltage-dividing capacitor C1 couples in parallel with a resistor Rlxc.To provide an ability to directly detect valleys as well aszero-crossings, divided drain voltage lxc is shifted by 90 degreesthrough a capacitor C2 to form a phase-shifted voltage lxc2. Thisphase-shifted voltage is AC coupled onto a DC voltage formed on avoltage divider node 405 between a pair of voltage dividing resistors R1and R2 that couple between a power supply node 410 for a power supplyvoltage VDD and ground. For example, resistors R1 and R2 may each havean equal resistance so that the DC voltage for node 410 is VDD/2. Asdivided voltage lxc resonantly oscillates, phase-shifted voltage lxc2oscillates through the capacitive coupling provided by capacitor C2. Butcapacitor C2 phase shifts this oscillation by 90 degrees such thatvoltage lxc2 oscillates 90 degrees out of phase with divided voltagelxc. Another voltage divider formed by a pair of resistors R3 and R4coupled between node 410 and ground to form a reference voltage (bias).Resistors R3 and R4 have the same dividing ratio as resistors R1 and R2so the reference voltage equals the DC common-mode value for voltagelxc2. A comparator 310 compares the reference voltage to voltage lxc2 todirectly detect valleys and zero-crossings for the drain-to-sourcevoltage for power switch transistor M1.

It has been contemplated that in some embodiments, ZVS conditionidentification may be implemented as follows: in case of using ZCDdetector and type I valley detector together, we identify insufficientenergy condition by receiving valley signal and sufficient energycondition by receiving ZCD signal; in case of using ZCD detector only,we identify insufficient energy condition by not receiving ZCD signalwithin the watchdog time and sufficient energy condition by receivingZCD signal; in case of using type I valley detector, we identifyinsufficient energy condition by receiving valley signal and sufficientenergy condition by not receiving valley signal within watchdog time;and in case of using type II valley detector, we always receive signal(covers both zero crossing and valley triggers) from the detector. Wecompare the arriving moment of the signal in two adjacent switchingcycles and we use the time gap to measure the time difference betweenthe two moments. (the gap concept is very important here). If the gap issmall, which means the detector is triggered after almost the sametransient time, and that is corresponding to a valley point(insufficient energy condition), because when valley occurs, thetransient is always close to half resonant period. If the gap is large,which means the detector is triggered much earlier than the previousswitching cycle, that means the voltage hits ground and terminate theresonance before it reaches valley point.

In some embodiments, the on-time control of the main switch may beimplemented as follows. Whenever ZCD or valley signal is received, weturn on the main switch. But in some case as descripted above, wewouldn't receive the detector signal, for example if we use only ZCD buthave insufficient energy, we will never receive the ZCD signal. So inthe real application, for example with ZCD only, after we have waitedfor ZCD signal for a while, we need to decide whether we should keepwaiting or we have been waiting for long enough time and should turn onthe main switch immediately before the voltage resonates back. We usewatchdog, and the watchdog time is adaptively adjusted according to thesignal receiving time in the previous switching cycle. Thus we can avoidturning on the main switch too late, which may make the operation offfrom the exact ZVS point too much.

Though the ZVS energy originally comes from the primary winding current(in the previous switching cycle), in the normal operation of ZVSswitching, we always firstly turn on aux switch for the optimal adaptiveon-time period, then turn aux switch off to result in ZVS of the mainswitch, then the main switch turns on to develop primary windingcurrent. In ZVS control, both energy and information is collected fromthe previous switching cycle and the adjustment is made for the currentswitching cycle.

As those of some skill in this art will by now appreciate and dependingon the particular application at hand, many modifications, substitutionsand variations can be made in and to the materials, apparatus,configurations and methods of use of the devices of the presentdisclosure without departing from the scope thereof. In light of this,the scope of the present disclosure should not be limited to that of theparticular embodiments illustrated and described herein, as they aremerely by way of some examples thereof, but rather, should be fullycommensurate with that of the claims appended hereafter and theirfunctional equivalents.

1. A controller for a switching power converter, comprising: avalley-mode detector configured to detect valleys in a drain voltage fora power switch transistor, each valley having a positive voltage,wherein the power switch transistor is configured to conduct a positivecurrent through an inductive storage element while the power switchtransistor is on; a zero-crossing detector configured to detectzero-crossings in the drain voltage; and an adaptive on-time controlcircuit configured to adjust an on-time period for an auxiliary switchtransistor between relatively-short periods that produce only valleys inthe drain voltage and relatively-long periods that produce onlyzero-crossings in the drain voltage to determine an optimal on-timeperiod at a boundary between the relatively-long periods and therelatively-short periods, wherein the auxiliary switch transistor isconfigured to force a negative current through the inductive storageelement when the auxiliary switch transistor is switched off.
 2. Thecontroller of claim 1, further comprising: a power switch driverconfigured to switch on a power switch at a conclusion of the optimalon-time period the auxiliary switch transistor.
 3. The controller ofclaim 1, wherein the auxiliary switch transistor is an active clampswitch transistor in a flyback converter.
 4. The controller of claim 1,wherein the auxiliary switch transistor is a synchronous rectifierswitch transistor in a flyback converter.
 5. The controller of claim 1,wherein the auxiliary switch transistor is a low-side switch transistorin a buck converter.
 6. The controller of claim 1, wherein the adaptiveon-time control circuit is configured to shorten the adaptive on-timeperiod until the valleys are detected.
 7. The controller of claim 1,wherein the adaptive on-time control circuit is configured to lengthenthe adaptive on-time period until the zero-crossings are detected. 8.The controller of claim 1, wherein the valley detector comprises: afirst voltage divider for dividing a drain voltage of a power switchtransistor into a divided drain voltage carried on a divided drainvoltage node; a second voltage divider for biasing a voltage dividernode with a DC bias voltage; a capacitor coupled between the voltagedivider node and the divided drain voltage node; and a first comparatorconfigured to compare a voltage for the voltage divider node to the DCbias voltage to detect peaks and valleys in the drain voltage of thepower switch transistor.
 9. The controller of claim 1, wherein the zerocrossing detector comprises a comparator configured to compare a drainvoltage for a power switch transistor to ground. 10-18. (canceled)
 19. Acontroller for a switching power converter, comprising: a valley-modedetector configured to detect both zero crossings and valleys in a drainvoltage for a power switch transistor, each valley having a positivevoltage; and an adaptive on-time control circuit configured to adjust anon-time period for an auxiliary switch transistor betweenrelatively-long periods that produce only valleys in the drain voltageand relatively-short periods that produce only zero-crossings in thedrain voltage to determine an optimal on-time period at a boundarybetween the relatively-long periods and the relatively short periods.20. A controller for a switching power converter, comprising: a zerocrossing detector to detect zero-crossings in a drain voltage of a powerswitch transistor; and an adaptive on-time control circuit configured tovary an on-time period for an auxiliary switch transistor fromrelatively-short periods that produce zero-crossings in the drainvoltage to relatively-long periods that do not produce zero-crossings inthe drain voltage and to determine an optimal on-time period at aboundary between the relatively-long periods and the relatively shortperiods.